This invention relates to display systems for analog values, and particularly to bar graph analog displays of a value which may vary and must be continuously monitored.
In industrial processes it is frequently necessary continuously to measure and monitor conditions such as temperature, pressure, flow, etc. which are critical to the process. Frequently, transducers convert the parameter to be measured to an electrical value which is then displayed by a D'Arsonval meter movement. Ideally, in a D'Arsonval meter movement the displacement of a pointer is proportional to the magnitude of an input current. This current is a known function of the input parameter to be measured so that the pointer displacement is a measure of the input parameter. Thus, the information is displayed in the form of a displacement which is continuously analogous to the input parameter.
Another electronic device which generates a displacement analog of an electrical value transduced from a parameter to be measured is a servo indicator. Here a motor drives an indicator and an electrical equivalent of the motor position is compared with the input value. When the motor reaches a null balance the indicator displays a value corresponding to the parameter to be measured.
Process parameters may also be displayed digitally. A multi-digit number is displayed which indicates the value of the input parameter.
All of these displays have certain disadvantages. Both the D'Arsonval movement and the servo system involve the use of moving parts which are susceptible to destruction or failure as a result of shocks. Servo indicators are subject to hysteresis errors. An all-electronic digital indicator is rugged and has no moving parts. However, it requires mental calculation on the part of the observer to determine how far the value has strayed from an ideal value or from between desired limits.
Analog displays yield more information about a parameter than just its magnitude. By observing the pointer or scale it can be noted that the variable is steady or drifing up and down and that it is, or is not, near some particular reference point. This type of information is generally called "rate information" or "trend information." Trend information is particularly useful when many instruments are grouped together, all of which are monitoring different parameters of a process. By viewing the magnitudes and trends of all measurements, a clearer picture of the process can be reached than by observing single measurements only. Furthermore, in many cases, it is desirable to know if a measured parameter is close to a danger point. By providing an index mark, it is possible to spot an approaching problem quickly using trend information.
Recently a new type of display device has been developed and marketed under the trademark Self-Scan. Basically this is composed of a multi-element gas discharge device in which the area of illumination is moved around a display by selective excitation of the elements. A large number of cathode elements are printed on an insulating substrate. A transparent cover for this pattern carries a transparent anode surface on its underside. The interior between the cover and the pattern is filled with an ionizable gas. A glow discharge is generated between the anode plate and the cathode elements. A keep-alive anode forms a continuous discharge with a keep-alive cathode. The gap between them provides a continuous source of metastable ions. The system is energized so that the glow at any electrode can transfer only between adjacent gaps. A reset cathode near the keep-alive gap transfers the glow to a reset gap formed between the reset cathode and the transparent anode. The remaining cathodes are connected in three groups so that the glow can be constrained to travel along the display by opening the cathode which has the glow and grounding the next cathode in the desired phase sequence. The length of the illuminated area represents a quantized indication of an input parameter having trend information. Such devices have a number of disadvantages. They merely display a process parameter without displaying the range within which this process parameter may vary. While the variable range may be painted behind the bar, it is desirable that the limits of this range be made variable so that they can be reset.
An object of the invention is to improve systems of this type.
Another object is to improve the display device itself so that it is capable of furnishing variable range limit information.